Charge transporting elements generally comprise a support and a charge transport layer across which charge moves under certain conditions. Charge transporting elements include electrophotographic charge generating elements.
In the use of such charge generating elements (also known as electrophotographic elements), incident light induces a charge separation across the various layers of the element. The electron and hole of an electron-hole pair produced within a charge generating layer separate and move in opposite directions to develop a charge between an electrically conductive layer and an opposite surface of the element. The charge forms a pattern of electrostatic potential (also referred to as an electrostatic latent image). The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer and the toner image is then fused to a receiver material. If desired, the latent image can be transferred to another surface before development or the toner image can be transferred before fusing.
The requirements of the process of generating and separating charge place severe limitations on the characteristics of the layers in which charge is generated and holes and/or electrons are transported. For example, many such layers are very soft and subject to abrasion. This places severe constraints upon the design of charge generating elements. Some configurations cannot provide a reasonable length of service unless an abrasion resistant overcoat layer (also known as "overcoat") is provided over the other layers of the element. This presents its own problems since charge must be able to pass through the overcoat.
The resistivity of an overcoat has major consequences in an electrophotographic system. If the overcoat has high resistivity, the time constant for voltage decay will be excessively long relative to the processing time for the electrophotographic element and the overcoat will retain a residual potential after photodischarge of the underlying photoreceptor. The magnitude of the residual potential depends upon the initial potential, the dielectric constants of the various layers, and the thickness of each layer.
A solution to this problem has been to reduce the thickness of the overcoat. Another solution is to provide an overcoat that is conductive. The overcoat must, however, not be too conductive. The electrophotographic element must be sufficiently electrically insulating in the dark that the element neither discharges excessively nor allows an excessive migration of charge along the surface of the element. An excessive discharge ("dark decay") would prevent the formation and development of the electrostatic latent image. Excessive migration causes a loss of resolution of the electrostatic image and the subsequent developed image. This loss of resolution is referred to as "lateral image spread" (evidenced as "image width"). The extent of image degradation will depend upon processing time for the electrophotographic element and the thickness and resistivities of the layers. It is thus desirable to provide an overcoat that is neither too insulating nor too conductive.
Silsesquioxanes are siloxane polymers, sometimes represented by the formula (RSiO.sub.1.5).sub.z, that are commonly prepared by the hydrolysis and condensation of trialkoxysilanes. Some of the polymers have been modified by the inclusion of polyethers or polydialkyloxysilanes. Generally, coatings of such materials are between 0.5 to 10 .mu.m thick and are applied from aqueous alcohol solvent systems. They have been commercially available from a number of sources for years (for example from Dow Corning, General Electric and Optical technologies). A number of patents describe the use of such polymers to provide abrasion-resistant coatings for various purposes [see for example U.S. Pat. No. 4,027,073 Clark), U.S. Pat. No. 4,159,206 (Armbuster et al), U.S. Pat. No. 4,277,287 (Frye), U.S. Pat. No. 4,324,712 (Vaughn, Jr.), U.S. Pat. No. 4,407,920 (Lee et al) and U.S. Pat. No. 4,923,775 (Schank)]. Typical uses of such polymers include scratch resistant coatings on acrylic lenses, photoreceptors and transparent glazing materials, and as overcoats for electrophotoconductive elements. For example, U.S. Pat. No. 4,159,206 (noted above) describes the use of neutral-charged, durable coating compositions that include colloidal silica and a mixture of dialkyldialkoxysilanes and alkyltrialkoxysilanes in a methanol/water solvent system. The mixture of silanes is believed to react to form silsesquioxanes.
Solid electrolytes (also known as solid ionic conductors) are solid materials in which electrical conductivity is provided by the motion of ions not electrons. A variety of solid electrolytes are inorganic crystals. Others are complexes of organic polymers and salts, such as complexes of poly(ethylene oxide) and alkali metal salts [see for example, Cowie et al, Annu. Rev. Phys. Chem., Vol. 40, (1989) pp. 85-113, Shriver et al, Chemical and Engineering News, Vol. 63, (1985) pp. 42-57, Tonge et al, Chapter 5 Polymers for Electronic Applications, ed. Lai, CRC Press, Boca Raton, Fla., 1989, pp. 157-210, at 162, and Cowie, Integration of Fundamental Polymer Science and Technology, Vol. 2, Elsevoir Publisher, New York, 21.5 (1988), pp. 54-62].
Electrical surface conductivities for polymeric and inorganic solid ion conductors are in the range of about 1.times.10.sup.-8 to 10 (ohms/square).sup.-1 [Surface conductivity is equal to conductivity divided by thickness and is expressed as (ohms/square).sup.-1 ]. Surface resistivity is equal to resistivity divided by thickness as expressed in ohms/square. For example, a resistivity of 1.times.10.sup.14 ohms-cm for a layer having a thickness of 5 .mu.m, equates to a surface resistivity of 2.times.10.sup.17. Solid electrolytes are used in rechargeable lithium batteries, electrochemical sensors, and display devices.
It has been important that any silicon overcoats in electrophotographic charge generating elements be adequately adhered to underlying layers such as photoconductor charge generating layers. Workers in this field have attempted to provide that adhesion in a number of ways. For example, U.S. Pat. No. 4,413,088 (Frye) describes the use of organic solvents that etch the underlying layers. Thermoplastic acrylic polymers are described as primer materials in U.S. Pat. No. 4,239,798 and U.S. Pat. No. 4,210,699 (both Schroeter et al) to provide adhesion to polycarbonates. Other primer compositions are described in U.S. Pat. No. 4,197,335 (Goossens) to adhere organosiloxane coatings to polycarbonates.
Polymeric emulsions are preferred as primer compositions over the organic solvent-based compositions because the emulsions are composed primarily of water that will not damage polycarbonate surfaces. Additionally, the viscosity of the compositions is relatively low even with high molecular weight acrylate polymers. Yet U.S. Pat. No. 4,439,509 (Schank) and U.S. Pat. No. 4,595,602 (Schank) describe the use of organic solvents for coating acrylics and other polymers in primer layers.
U.S. Pat. No. 4,407,920 (Lee) teaches the use of a conductive primer in electrophotographic elements in order to maintain a low residual potential when the photoconductor is overcoated with a silicone resin. The low residual potential is desirable to produce images of high density and low background.
More recently, U.S. Pat. No. 5,693,442 (Weiss et al) and U.S. Pat. No. 5,731,117 (Ferrar et al) describe the use of silsesquioxanes in glassy solid electrolyte layers that are used as overcoats in electrophotographic charge generating elements. They also describe the use of primer layers between the charge generating layer and the solid electrolyte layer. Disclosed primer materials include vinyl polymers such as a poly(methacrylate-co-methylmethacrylate-co-methacrylic acid) latex and poly(vinyl pyrrolidone-co-methacrylic acid). In addition, U.S. Pat. No. 5,731,117 describes the primer layer composition as further including TRITON X-100 nonionic surfactant. This surfactant includes poly(ethylene oxide) moieties that are conductive in aqueous solutions.
Despite the advances provided by the inventions in the noted Ferrar et al and Weiss et al patents, there is a need to reduce the lateral image spread ("image width") even more, especially when the noted elements are used in electrophotography at high relative humidity.